Effects of orally administered hormonal contraceptives on the musculoskeletal system of healthy premenopausal women—A systematic review

Abstract Introduction The musculoskeletal system (MSK) is one of the extragonadal target tissues of sex hormones: osteoblasts and osteocytes express estrogen receptors, while in fibroblasts of the anterior cruciate ligament (ACL) and myocytes of the vastus lateralis muscle (MVL), estrogen and progesterone receptors can be detected by immunoassay. Indeed, upon binding of sex hormones to the extragonadal receptors, the MSK seems to respond to varying levels of sex hormones with structural adaptation. Hormonal contraceptives can affect the musculoskeletal system; however, there is a lack of high‐quality studies, and no recommendation for female athletes exists. Material and Methods This is a systematic review of publications on the effects of oral hormonal contraceptives on the biomechanical properties of tendons, muscles and ligaments, muscle strength, and soft tissue regeneration. A systematic database search was performed using MESH keywords and PRISMA (Preferred Reporting Items for Systematic Reviews and Meta‐Analyses) methodology in Pubmed and Cochrane to identify studies investigating the influence of oral hormonal contraceptives on muscles, tendons, and ligaments of healthy, adult, premenopausal women. The risk of bias in the studies included was assessed by two independent researchers using the ROBINS‐I Tool. Results Nine comparative studies were identified that met the inclusion criteria. Endpoints were muscle strength and biomechanical tissue properties. No significant influence of oral hormonal contraceptives on muscle strength was found, although general muscle growth and Type I fiber growth were found to be significantly increased in a dose‐dependent manner. There was a negative effect on regeneration of muscle strength after exercise. The stiffness of tendons remained unchanged, while their size adaptation to load increased. Conclusion The anabolic effect could be beneficial for specific sports, whereas reduced muscle regeneration could be disadvantageous for women exercising with high‐performance demands. The different effects on tendons and ligaments and the functional consequences of altered ligament and muscle stiffness, especially with regard to synthetic hormones, should be further investigated.


| INTRODUCTION
The musculoskeletal system (MSK) is one of the extragonadal target tissues of sex hormones: osteoblasts and osteocytes express estrogen receptors, while in fibroblasts of the anterior cruciate ligament (ACL) and myocytes of the vastus lateralis muscle (MVL), estrogen and progesterone receptors can be detected by immunoassay. 1,4 Indeed, upon binding of sex hormones to the extragonadal receptors, the MSK seems to respond to varying levels of sex hormones with structural adaptation. 1,4 A higher estrogen concentration in the ACL was reported to be associated with a 40%-50% reduction in collagen synthesis and significantly reduced fibroblast proliferation 5,6 while exogenously administered estrogen after mechanical stress enhanced activation of satellite cells and proliferation of myoblasts in the MVL and in the soleus muscle in a mouse model. 7 Furthermore, an inhibiting effect of oral contraceptives (OCs) on the synthesis of myofibrillary proteins in human muscles was detected. 8 At once, hormonal contraceptives do not have an influence on the smooth muscle in arterial vessels and the menstrual cycle might influence endothelial function in major vessels. 9,10 Injury prevalence seems to vary through the menstrual cycle; however, results are inconclusive and the evidence is weak. 11,14 In this context, musculoskeletal and neurophysiological changes associated with varying hormone levels such as neuromuscular activation, joint laxity, postural control, or muscle strength are discussed as risk factors. 6,11,15,16 Anatomical differences between women and men also need to be considered, as a larger Q-angle in women can also contribute to a higher ACL injury risk. 17 Konopka et al. 18 performed a systematic literature review for studies investigating the effect of OCs on the risk of soft tissue injuries and tissue laxity. While they identified 29 studies, only three were found to provide high-level evidence. Overall, there are still conflicting findings in the current literature, 18,21 and well-performed studies meeting the gynecological endocrinology criteria are lacking. 22 Given that no evidence-based recommendation exists regarding contraceptive methods for young female athletes, this systematic review was performed to address the research gap of OCs and their influence on the MSK with a focus on muscle strength and soft tissue changes and regeneration, which are important predictors for a higher risk of injury. 11,12 2 | MATERIALS AND METHODS

| Electronic database search
In preparation of the literature search, we refined our research question in relation to population, intervention, control group, endpoints, and inclusion and exclusion criteria. The PICO (population, intervention, control, and outcomes) scheme was used for structuring the search process. 23 Relevant studies were identified by an electronic literature search in PubMed and Cochrane online databases in June 2020. The search terms from the categories intervention and endpoints were combined as shown in Figure 1.
An exemplary search combination was accordingly "contraceptive agents" [MeSH Terms] AND "muscle strength" [MeSH Terms]." In the course of the search, each search term from the "Intervention" category was combined with each search term from the "endpoints" category. All search results in English and German language, regardless of the year, were considered and supplemented by manual searches in specialist journals. PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) methodology was applied.

| Inclusion and exclusion criteria
The inclusion and exclusion criteria for the selection of relevant studies were determined based on the PICO scheme as follows: (1) In terms of the study population, we included studies that investigated healthy women from the age of 18 up to the onset of menopause. Exclusion criteria were studies of animal models, minors, postmenopausal women, or subjects not expressly defined as healthy.
(2) With regard to the intervention and control groups, studies were included in which a group of healthy women who took oral hormonal contraceptives in defined doses was compared with a female control group who did not use any hormonal contraceptives.
Exclusion criteria were a lack of information on the dosage of the preparations taken, excessive doses, that is, an ethinyl estradiol (EE) content of more than 50 µg, hormonal contraceptives that were not administered orally, and a lack of a control group without taking hormonal contraceptives.
(3) With regard to the endpoints, studies were included that investigated the effects of oral hormonal contraceptives on the biomechanical properties of tendons, muscles and ligaments, muscle strength, and soft tissue regeneration within the groups compared.
Studies that investigated other endpoints and, for example, dealt with the influence of oral hormonal contraceptives on bone density were excluded.

| Selection process
The systematic database search identified a total of 556 potentially relevant studies. The selection process is shown in Figure 2.
Reasons for study exclusion after full-text screening are provided in Figure 3. Note that the high proportion of studies was excluded because no dose information on the contraceptives taken was provided.
Failure to explicitly state that the test subjects were healthy also led to numerous exclusions. The exclusion criterion of "inhomogeneous test group" (see Figure 2) relates to the diversity of physical activities in the study populations of Pokorny et al., 21 which could be a disruptive factor.
The risk of bias in the included studies was assessed by two independent researchers using the ROBINS-I Tool 24 and is shown in

| Characteristics of the included studies
The publications included are comparative studies in which a group of test subjects taking OCs was compared with a group of test subjects who did not take hormonal contraceptives. 25,33 Ekenros et al. 3 performed a cross-over study, comparing phases with and without the intake of combined OCs (COCs). For this purpose, after the first measurement cycle, the subjects who were taking COCs at the start of the study stopped taking them and the subjects who were not using hormonal contraceptives at the start of the study started taking COCs at a dose comparable to that of the first group. 26 Mackay et al. 31 divided the control group according to the cycle phase into a subgroup in the follicular phase and a group in the ovulation phase. The endpoints of the included studies were muscle strength and/or properties of muscles, tendons, or ligaments; the level of significance was p < 0.05. F I G U R E 1 Flowchart of the research process in online database research. PICO, population, intervention, control, and outcomes. Table 1 lists important characteristics of the study populations. Table 2 summarizes the methodological characteristics and results of the studies identified to be relevant for the purpose of our literature review. In six of the studies, the subjects of the groups compared in the original studies had a further intervention; these are also shown in Tables 2 and 3. Table 4 summarizes the risk of bias classifications for all studies included. The risk was assessed using the ROBINS-I Tool. 24 More detailed information on the risk assessment procedure can be found in the Supporting Information: Material.

| Effect on hormonal fluctuation
Several studies consistently showed that COC intake significantly reduced estrogen and progesterone levels and their fluctuations. 25,29,31

| Effects on muscle strength
Six studies investigated the influence of COCs on muscle strength. 25 on muscle strength nor on its increase through a training program. 25,27,30,31,33 However, Dalgaard et al. 25 and Romance et al. 33 showed an anabolic effect on general muscle growth as well as on Type I fibers, while Mackay et al. 31 detected a negative effect of COCs on regeneration of muscle strength after exercise and muscle soreness. Creatine kinase increased with COC intake, which correlated with greater pain. 31,34 However, a severe risk of bias for measurement was found in four studies. 26,27,31,32 Overall, female athletes may use COCs with an estrogen dose of <50 µg without suffering from any disadvantages in terms of strength performance.

| Effects on the texture and elasticity of muscles, tendons, and ligaments
Five studies investigated the influence of COCs on morphology and mechanical properties of muscles, ligaments, and tendons. 25,28,30,32 The stiffness of the patellar tendon was not significantly affected by COCs. 28 Structural differences were only detected by Dalgaard et al., 25 with a higher content of crosslinks in tendons in COC-taking women. There were no significant group differences in collagen content. 28 Hansen et al. 5,8 showed reduced insulin-like growth factor-1 messenger RNA or protein expression during COC intake. The tendon cross-sectional area correlated significantly with COC intake. 28 The ACL was generally more rigid during COC intake; however, less markedly so after warm-up. 29 Lee et al. 29

| Musculoskeletal injury risk and OCs
ACL stiffness was increased when COCs were taken, as was the cross-sectional area increase of the patellar tendon in response to loading. 28,30 However, no significant differences in patellar tendon stiffness were reported. 28,30 Whether the musculoskeletal adaptations observed in women  18 For the endpoint of injury risk, two of their included studies rated with a high level of evidence suggested a protective effect of OCs on ACL injuries. 18 The protective effect may be related to changes in collagen turnover, as Hansen et al. 28 Table 2 2. Biopsy: -Trend: Crosslinks in patellar tendon in OCG > CG (p = 0.07) 3. MRI: -Cross-sectional area of the patellar tendon was significantly increased compared to baseline (p < 0.05) with no difference between groups in the response to training

| Influence of OCs on muscle and tendon stiffness
Although none of the studies included in our review had the occurrence or risk of MSK injuries as endpoint, the publications discuss possible consequences of the effects of COCs on tissue stiffness. 29,30,32 In the study of Hansen et al., 28 the estrogen level in the control group correlated inversely with patellar tendon stiffness. Across both groups, irrespective of OC intake, estrogen levels tended to correlate with deformation and strain of the patellar tendon, suggesting a higher risk of injury. 28 Lee et al. 29 also reported the fluctuation in ACL stiffness during the menstrual cycle to show an inverse correlation to the estrogen level. They assume the higher ACL stiffness observed in women taking COCs to potentially reduce the risk for ACL tears. 29 Overall stiffness of the knee joint, measured as the force exerted to flex and extend the knee joint, was higher when COCs were taken. 29 Ligament stiffness was lower when heat was applied, especially in the group taking COCs. 29 Heat eliminated the inversely correlating fluctuation of ACL stiffness with the estrogen level, suggesting that the higher body temperature around ovulation may contribute to differences between OC and non-OC users. 29 Morse et al. 32  ambitious leisure or competitive sports, this aspect could be a limiting factor for training motivation and performance. Gynecologists and sports medicine physicians need to take this into account. 22

| Measurement methods
As exact reporting of the measurement procedure is essential for studies of the MSK, authors should both describe the measurement method and the examiners' experience. In terms of the measurement method, four of the studies included in our systematic review were found to be severely biased. 26,27,31,32 Romance et al. 33 assessed muscle growth by determining the lean mass, a method that is prone to error due to changes in water content. Specifically, the determination of tissue stiffness and practicability of the measurement method turned out to be difficult. 33 Lee et al. 29 is carried out manually in everyday clinical practice using the socalled Lachmann test, which does not allow precise quantification. 39,40 In contrast, Hansen et al. 8 calculated patellar tendon stiffness from dynamometric and sonographic measurements, which is a far more direct but also far more time-consuming method to measure ligamentous stiffness and thus challenging to integrate into clinical practice. Ultrasound elastography could offer a noninvasive and easy-to-use alternative measurement method here. 41 While it is currently used in various specialist areas such as quantification of liver fibrosis or breast cancer detection, ultrasound elastography can also be applied to quantify the elasticity of muscles, tendons, and ligaments. 42,44 Another alternative is the MyotonPro, a portable device for the noninvasive measurement of mechanical tissue properties. 37 It detects changes in tissue stiffness by emitting a mechanical impulse and recording the resulting tissue oscillation. 37,45

| Differences in endogenous and synthetic hormones
It is well known that OCs affect the natural hormonal fluctuation. 46 In addition, synthetic hormones, such as EE, do differ in receptor affinity compared to endogenous hormones, 47 which needs to be considered in the research of athletes using COCs in comparison with naturally menstruating women. 22 Differences in hormone profiles between COC users and non-COC users in the examined studies were to be expected, as the cycle phases are suppressed in COC users, which results in a negative feedback reaction and a lower endogenous estradiol concentration. 22 Furthermore, four studies included COCs with different progestins 25,28,33 and two studies did not report the type of progestin, 29,30 limiting the interpretation of the results.
Future studies need to consider a correct determination of the cycle phases and should examine COC users with the same preparation to avoid inconclusive results. 22

| Limitations
Consistent use of narrow inclusion and exclusion criteria with regard to population and intervention resulted in an overall homogeneous study population under precisely defined estrogen influence, which is quite representative in relation to the age group of users of OCs. 48 In future studies, a restriction of progesterone content might be defined in the inclusion criteria, since progesterone effects on the MSK are less well understood compared with the estrogen and gestagen content of COCs. 25,27,49 Overall, a small population size of the studies is a weakness. Another limitation was found for Lee et al. 30 and Morse et al., 32 whose measurements were performed without considering the phase of the menstrual cycle and without measuring sex hormone levels. This is a disruptive factor given the strong fluctuations during the menstrual cycle. 10,12,22,30,32 For future studies, it is fundamental to adhere to specific criteria, such as an exact definition of cycle phases, and a specialist in gynecological endocrinology should be involved in the study design. 22 Numerous relevant publications had to be excluded because they provided no dosing information. In future work, the preparations taken as well as the duration of intake and the cycle history should be precisely surveyed and documented. 22 These facts were missing in Ekenros et al. 26 and Mackay et al. 31

| CONCLUSION
The studies included in this literature review reported no effect

ACKNOWLEDGMENTS
The authors thank Ms. Bettina Herwig for the language editing of the manuscript. Open Access funding enabled and organized by Projekt DEAL.

CONFLICT OF INTEREST
Markus H. Lerchbaumer reports having received consultancy honoraria from Canon Medical Imaging and Siemens Healthineers.
The remaining authors declare no conflict of interest.

DATA AVAILABILITY STATEMENT
The authors confirm that the data supporting the findings of this study are available within the article and its supplementary materials.

ETHICS STATEMENT
The systematic literature search was carried out in compliance with Charité's statutes aimed at ensuring good scientific practice.

TRANSPARENCY STATEMENT
Claudia Römer affirms that this manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned have been explained.